A plunger solenoid is a device that includes an electrically energizable coil wound on a non-magnetic form within which a magnetic plunger may move. A solenoid includes a mechanical stop or butt to restrict plunger movement. The stop or butt is made of a magnetically permeable material. The non-magnetic form or spool, electrically energizable coil, plunger and mechanical stop are surrounded by a ferromagnetic casing such as steel that is formed of two parts. The casing includes a generally cylindrical element that surrounds the solenoid element and a pole piece. The plunger butt and pole piece are made of soft magnetic materials that can retain varying degrees of residual magnetism depending upon their composition. Since the solenoid contains no permanent magnetic field, the magnetic field is produced only when the coil is energized. When the coil is energized by passing an electrical current therethrough, a magnetic field is produced in and around the core volume within which the plunger is positioned. The casing, plunger, butt and pole piece together form a magnetic circuit which intensifies the magnetic flux in the air gaps between the plunger and the butt as well as between the plunger and the pole piece. Because of the magnetic field in the core volume, the movable plunger is pulled toward a central position within the coil. The more intense the magnetic field in the gaps between the plunger and the butt and between the plunger and the pole piece, the greater the force on the plunger.
Solenoids are widely used for operating circuit breakers, track switches, valves and many other electromechanical devices. Thus, the movable plunger may be attached to any one of variety of mechanical elements such as a seat of a valve, the movement of which can be utilized to control flow of gases or liquid through the valve. In use, as the moving plunger approaches the butt, the mechanical force of the moving plunger increases rapidly due to a decrease in the reluctance of the magnetic flux path. The plunger strikes the butt with maximum force thereby creating noise, vibrations and chattering in the solenoid. A significant problem associated with solenoids is that they tend to generate noise, caused by the plunger striking the butt and by the plunger rubbing against the walls of the core defined by the interior surface of the spool. The impact force against the butt and the frictional force against the core walls create wear particles which can cause wear on the plunger and on the spool which, in turn, limit the life of the solenoid. Typically, the plunger displacement is small such as less than 1 mm and the radial clearance between the plunger and the core wall is about 0.1 mm. In addition, the clearance between the pole piece and the plunger is also about 0.1 mm. Since there is no alignment mechanism for the plunger within the solenoid, the plunger may scrape the walls of the core, causing undesirable wear.
Noise generated by solenoid devices such as solenoid valves pose serious restrictions in their use in apparatus that must perform quietly. For example in medical applications such as dialysis machines, blood chemistry instruments, blood pressure monitors and ventilators/respirators, it is necessary that valves be quiet to assure patient comfort. Presently this is achieved by placing excessive acoustic foam insulation around the apparatus, which renders the apparatus large and bulky and therefore undesirable.
Ferrofluids are magnetically responsive materials and consist of three components: magnetic particles, a surfactant and a liquid carrier. The particles, typically Fe.sub.3 O.sub.4, are of submicron size, generally about 100 .ANG. in diameter. The magnetic particles are coated with a surfactant to prevent particle agglomeration under the attractive Van der Waals and magnetic forces and are dispersed in the liquid carrier. Ferrofluids are true colloids in which the particles are permanently suspended in the liquid carrier and are not separated under gravitational, magnetic and/or acceleration forces. The liquid carrier can be an aqueous composition, an oil composition or an organic solvent composition.
Ferrofluids are helpful in that they eliminate or substantially reduce the noise associated with solenoid action. In order to return the plunger to its original position before energizing the magnet used to drive the plunger, a primary spring is used to push the plunger back when the electromagnet is turned off. The travel of the plunger is typically much less than the compressible range of the primary spring. Thus, during the travel of the plunger, the force of the primary spring is relatively constant compared with the magnetic force, which varies greatly over small changes in the plunger position. The difference between the magnetic force and the primary spring force increases dramatically with the decrease of the gap between the plunger and the gap. When the gap approaches zero, the primary spring force is very weak compared to the magnetic force.
This relative weakness of the primary spring force at small gap distances causes undesirable effects to the performance of the solenoid valves. For example, during the operation of the valves, certain liquids can be present at the plunger/butt interface. The liquid can either come from the working agent the valve is controlling, or a lubricant, or a noise reduction agent, such as a magnetic ferrofluid previously discussed. Once the plunger and the butt become close together, the viscosity and surface tension effects of the liquid at the interface tend to keep the plunger from moving away from the butt. A weak spring force may greatly extend the time needed to push-back the plunger, resulting in a slow de-energizing response time that is undesirable in many applications.
Accordingly, it would be desirable to provide solenoids that can be operated with a quick de-energizing response time, a time faster than the spring force of the primary spring would otherwise allow.